Diffuse reflectance spectroscopy has been investigated for the early diagnosis of epithelial cancer. Due to its non-invasiveness and capability to provide quantitative information about the physiological and pathological status of tissues in real time, this technique of diffuse reflectance spectroscopy has significant potential to be widely used in clinical settings.
In a typical diffuse reflectance setup, a fiber-optic probe serves as the conduit for the delivery of illuminating light and collection of emitted light. The fiber-optic probe is a metal cylindrical tube enclosing one or multiple optical fibers, in which some fibers are used for delivering light onto a tissue surface and the same fibers or other fibers depending on the probe design are used for collecting light emanating from the tissue surface.
Although fiber-optic probes are widely used in optical spectroscopy due to their flexibility and high efficiency, the uncertainty in measurements due to inconsistent probe-sample pressure is difficult to remove. The inconsistent pressure may induce significant distortions in measured spectra, which may consequently cause large errors in diagnosis.
To reduce such an artifact, lens based setups for non-contact diffuse reflectance measurements have been explored. For example, a lens based setup involving a spherical and a flat folding mirror for illumination with two achromatic lenses for detection has been investigated. An illumination fiber is placed at the focal point of the spherical mirror to deliver the white light to a tissue surface. A detection fiber is placed at the focal point of the top achromatic lens to transmit diffusely scattered light to a spectrometer. The distance between the illumination and the detection area can be varied continuously; moreover, both source and detection fibers with different diameters can be used. Therefore, this non-contact lens based setup is able to perform spatially resolved diffuse reflectance measurements without physically contacting a tissue sample.
A different setup for non-contact diffuse reflectance measurements has also been proposed where two collimating lenses are used to image the illumination and collection fibers onto a tissue surface and serve as a non-contact probe to eliminate the influence of inconsistent probe-tissue pressure that may be present in a contact probe. A customized autofocus mechanism has been incorporated in the setup to address the limit of the lens in focal depth.
Further, a non-contact lens based setup for time-resolved diffuse reflectance measurements, in which laser scanning is used to achieve imaging, has been introduced.
Lens based setups have been demonstrated as promising tools for non-contact diffuse reflectance measurements without distortion due to inconsistent probe sample contact.
There also exists other techniques such as low coherence enhanced backscattering, diffuse backscattering, and confocal technique for non-contact diffuse reflectance measurements. The dependence of the penetration depth of low coherence enhanced backscattering signals on optical properties has been examined using Monte Carlo modeling, but the simulation of lens based illumination and detection has not been explored. In other words, there has been no report of Monte Carlo modeling of lens based non-contact setup for depth sensitive diffuse reflectance measurements.
Similar to diffuse reflectance spectroscopy, which involves the use of visible light, ultraviolet-visible fluorescence spectroscopy has also been widely explored in the detection of precancers in human epithelial tissues. Vital diagnostic information about morphological and biochemical changes can be extracted from various fluorophores present in epithelial tissues. Such autofluorescence spectroscopy offers a non-invasive and effective detection approach. However, the distribution, or more specifically, the depth distribution of endogenous fluorophores may be affected by the progression of the disease state and various other factors such as age and menopausal status.
A depth-sensitive probe that can measure fluorescence spectra as a function of depth enhances the diagnostic performance of this technique. Achieving high depth sensitivity to a specific subsurface region in human epithelial tissue is a great challenge as photons are quickly scattered when entering a tissue that is a diffusively scattering medium. Furthermore, the dominance of fluorescence signals from overlying layers significantly reduces the contrast of fluorescence signals originated from the subsurface region of interest, thus limiting the diagnostic performance of this technique.
Currently, depth-sensitive fluorescence measurements can be achieved in two approaches, namely the fiber optic-based contact setup and the lens-based noncontact setup.
Fiber optic setups achieve depth-sensitive measurements by varying the source-detector separation, aperture diameter, and/or angle of illumination and collection fibers.
Lens-based setups use a single lens or a combination of lenses to achieve a cone configuration, in which both the excitation and emission volumes form light cones in an optically transparent medium. One drawback of the lens-based setup is the limited sensitivity to fluorescence originating from sub-surface layers due to the contribution from shallower layers in a layered structure such as epithelial tissues.
There is therefore a need for an optical detection device and an optical detection method with improved depth sensitivity to deep layers of samples and without the need to perform alterations which may induce uncertainty in optical coupling and significant inconvenience in clinical measurements, thereby addressing at least one or more of the problems mentioned above.